1. Field of the Invention
The present invention relates to a stainless steel having excellent corrosion resistance to ozone added water such as ozone added ultrapure water used in semiconductor manufacturing processes and the like, as well as to a manufacturing method thereof.
2. Description of the Related Art
In the field of the manufacturing of semiconductors, the integration of devices has increased in recent years. In the manufacturing of a device called ULSIs, a fine circuit pattern of 1 .mu.m or less is required on substrates such as silicon wafers.
Adhesion of fine dust or impurity gas to such fine circuit patterns causes a circuitry problem. Therefore, in the ULSI manufacturing processes, various measures are taken to prevent such contamination.
For protection of substrates from contamination from the work environment, substrates are processed within a clean room. In order to maintain cleanliness of a clean room, not only air in the clean room must be filtered, but also gases and water used therein must be of high purity. Particularly, ultrapure water whose fine particles and trace impurity is normally used as pure water.
For those reasons, pipes and members used for such gases and water that have high purity is required for the inner surface thereof discharges as contaminants only minimum amount of particles and gases.
Conventionally, ferritic or austenitic stainless steels have been used as materials for pipes and piping members used in semiconductor manufacturing processes. Such stainless steels, when used for high-purity gases, must not emit particles therefrom and must not cause adhesion or adsorption of water. When the stainless steel used for passing ultrapure water therethrough, those stainless steels must be such that metallic ions are less likely to be dissolved.
To meet these requirements, the stainless steels to be in contact with high-purity gases or ultrapure water are usually subjected to a surface-smoothing process to thereby make their surface areas as small as possible. For example, the inner surface of a steel pipe for piping is smoothed, in many cases, so that the maximum height indicative of surface roughness as defined by JIS B0601 (hereinafter referred to as maximum roughness and represented by Rmax) becomes not greater than 1 .mu.m. For this smoothing process, electrochemical polishing is usually conducted on cold-drawn steel pipes and mechanically polished piping members. However, this electrochemical polishing method involves difficulty in controlling an electrolytic solution and conditions of electrolysis and is low in productivity, resulting in increased manufacturing cost of steels.
Also, even when a stainless steel having a smoothed inner surface is used, metallic ions of Fe, Cr, Ni, and other constituent atoms thereof may be dissolved therefrom with pure water such as ultrapure water and the like. In order to prevent this dissolution of metallic ions, various proposals have been made as described below.
A promising measure against the dissolution is to provide an oxide film or the like on the surface of the base metal of a stainless steel.
Japanese Patent Application Laid-open (kokai) No. 1-87760 discloses a stainless steel for use as a material for a semiconductor manufacturing apparatus whose electrochemically polished base metal surface is provided with an amorphous oxide film having a thickness of 75 angstroms or more. Also, Japanese Patent Application Laid-open (kokai) No. 1-180946 discloses a ferritic stainless steel pipe for ultrapure water which has a specific composition and whose inner surface is provided with a passive film having a maximum roughness (Rmax) of 5 .mu.m or less.
Furthermore, some of the present inventors propose in Japanese Patent Application Laid-open (kokai) No. 6-33264 an austenitic stainless steel for a high-purity gas which contains Ti (0.02 to 1.0% by weight) or Al (0.02 to 1.0% by weight) or both and whose base metal surface is smoothed to a maximum roughness (Rmax) of 1 .mu.m or less and provided with an oxide film mainly comprising a Ti oxide or an Al oxide or both.
Also, Japanese Patent Application Laid-open (kokai) No. 7-62520 discloses an austenitic stainless steel for use in a clean room whose base metal containing Si (0.5 to 5.0% by weight) is provided on the surface thereof with an oxide film mainly comprising of Si oxide.
Also, in Japanese Patent Application Laid-open (kokai) No. 7-60099, a steel for use in a super-high vacuum is disclosed wherein the base metal is a stainless steel containing Al (1 to 6% by weight) and a tight Al oxide film having a thickness of 10 to 150 angstroms is provided on the base metal surface thereof. In addition, the inventors of the present invention have proposed an austenitic stainless steel whose base metal has a specific composition and which is provided on the base metal surface thereof with an oxide film mainly comprising of Al oxide, thereby providing excellent oxidation resistance(Japanese Patent Application Laid-open (kokai) No. 6-271992).
Stainless steels employing the above-described measures are practically usable as materials for pipes and apparatus members for handling ultrapure water and high-purity gases as materials for use as well as at high temperatures.
Recently, ozone added water has been used to clean substrates such as silicon wafers in semiconductor manufacturing processes.
In semiconductor manufacturing processes, ultrapure water containing a surfactant, acid, alkali or some of them is normally used to clean silicon wafers and the like. However, a cleaning method using such cleaning water can clean off metallic substances, but does not perform well in cleaning off organic substances, particularly fats and oils which are relatively stable against chemicals. Also, a surfactant, acid, and alkali contained in cleaning water are impurities themselves. Thus, in order to clean off cleaning-water-induced impurities from a silicon wafer surface, "rinsing" must be conducted through use of ultrapure water having higher purity.
In order to omit this "rinsing" step, there has recently been attempted a cleaning method which uses ozone (O.sub.3) added ultrapure water to clean silicon wafers. As seen from its use as bleach and disinfectant, ozone has strong oxidation power, and thus ionizes metals and decomposes organic substances. Accordingly, in cleaning with ozone added ultrapure water, adhering metals are removed through ionization, and organic substances are removed through decomposition. Furthermore, after cleaning, ozone decomposes by itself and does not remain on silicon wafers as a contaminant. Thus, cleaning with ozone added ultrapure water can advantageously omit the "rinsing" step.
As described above, ozone added ultrapure water is quite effective for cleaning silicon wafers. However, cleaning with ozone added ultrapure water involves contamination of ozone added ultrapure water with contaminants from pipes and apparatus members in its feed system. Since ozone added ultrapure water corrodes stainless steels used as materials for pipes and apparatus members, metallic ions of Fe, Cr, Ni and the like are dissolved therefrom, they cause contamination for ozone added ultrapure water with such ions.
The aforementioned stainless steels developed for ozone-free ultrapure water and high-purity gases show substantially good performance in prevention of dissolution of metallic ions into ultrapure water as well as particle emission. However, since these stainless steels are not developed with the intention of being used with ozone added water, they are not practically usable with ozone added water due to dissolution of metallic ions of Fe, Cr, Ni and the like therefrom into ozone added water.
Furthermore, the aforementioned stainless steels proposed in Japanese Patent Application Laid-open (kokai) Nos. 1-87760 and 6-33264 require electrochemical polishing in the course of their manufacture, resulting in decreased productivity from electrochemical polishing and increased cost of manufacture.
In view of the foregoing circumstances, there arises the need for developing a stainless steel having excellent corrosion resistance to ozone added water and capable of being manufactured at low cost. These stainless steels are used in fields other than manufacturing semiconductors, for example, in the pharmaceuticals manufacturing fields which involve the handling of ozone added water.
Stainless steels have strength required of materials for pipes and apparatus members handling ultrapure water in semiconductor manufacturing processes and also have excellent workability. However, as described above, under the present conditions, they have a drawback of poor corrosion resistance to ozone added water.
An object of the present invention is to provide a stainless steel having excellent corrosion resistance to ozone added water which does not cause dissolution of metallic ions even when used as a member for ozone added water and which can be manufactured at low cost, as well as to provide a manufacturing method thereof.